At radio frequencies, superheterodyne receivers typically have sensitivities that are orders of magnitude higher than those of direct detection receivers. Such superheterodyne receivers, in addition to a signal source, usually require a separate local oscillator or pump source.
With certain constraints, superheterodyne receivers can be realized with just one source. The output from this source is divided and one of the component signals is Doppler shifted. The Doppler shifted component serves as the local oscillator source and the unshifted component serves as the signal source or vice versa. At submillimeter wavelengths, however, presently known electronic means of frequency shifting are very inefficient and produce very small output power levels.
Reflection of a signal from a metallic surface which is in linear motion will produce a constant Doppler shift of the reflected signal. As a practical matter, however, linear motion can only be sustained for limited periods of time. For a continuously Doppler shifted signal, some kind of repetitive process of linear motion must be employed.
Imperfections in presently known mechanical means for frequency shifting make it impossible to generate a single Doppler shifted frequency. Instead, a spectrum of frequencies is generated which has a frequency spacing which is the reciprocal of the period of the time repetitive process. This spectrum has a maximum amplitude at or near the frequency determined by the ratio of the velocity component of the reflector (or reflectors) in the direction of wave propagation and the wavelength. The spectral line of maximum amplitude is surrounded by other spectral lines whose amplitudes depend on the "smoothness" of the repetitive process.
A spectrum of frequencies, rather than a single frequency, is undesirable in certain applications, such as in radar applications, where they may lead to ambiguities. One way to eliminate this problem is to space the non-desirable lines far apart from the desired one. This requires a mechanical process of the highest possible repetition rate.
In an article entitled "High Sensitivity HCN Laser Interferometer For Plasma Electron Density Measurements" by D. Veron, which article appears in Volume 10, Number 1 of Optics Communications, dated January 1974, there is described a rotating mechanical frequency shifter which is, in effect, a large rotating paddle wheel. Each paddle reflects the signal over a small angle of rotation, where its motion can be considered as being linear. Then it is replaced by the next paddle. Although high Doppler offsets may be achieved with high tangential speed of the paddle, many paddles are required to satisfy the linear motion approximation. Consequently, the wheel will have a large diameter and a slow rate of rotation thus producing a close frequency spacing in the resulting spectrum.
In an article entitled "CW IR Laser Induced Chemistry, Isotope Separation and Related Laser Technology at NRL" by T. J. Mannucia, which article appears in Laser in Chemistry, pages 210-215, Elsevier Scientific Publication Company, dated 1977, there is described another mechanical Doppler frequency shifting scheme based upon multiple reflections between a stationary involute spiral cylinder and a multiplicity of mirrors attached to a concentrically rotating cylinder. The transmissive scheme as described by Mannucia, where radiation enters in an axial direction at one end of the cylinder and leaves at the other end of the cylinder, neglects the axial spreading between reflection points as reflections occur at increasing radial distances on the involute cylinder. In addition, it does not provide means to make the Doppler shifted signal phase coherent between cylinder revolutions and hence an undesirable spectral spreading will occur.
In U.S. Pat. No. 4,264,141 issued to Karl Guers et al on Apr. 28. 1981, there is disclosed an arrangement of apparatus for frequency shifting a monochromatic narrow bandwidth light beam. Unlike the Mannucia device and the present invention, the Doppler shifting curved surfaces are not involutes of circles. Like the Mannucia device, however, it is a transmissive type device, and not a device in which an input signal is reflected back upon itself, as is the case in the present invention. The Guers et al patent requires pairs of stationary and pairs of rotating reflectors in order to function. A continuous wave device employing several pairs of Doppler shifting mirrors would therefore be physically large in comparison to the present invention. Furthermore, no provisions have apparently been made for a phase-continuous transition from one frequency shifting contour to the next, which is essential in narrowing the output spectrum.